1. Field of the Invention
This disclosure relates to driving LED (Light-Emitting Diode) lamps and, more specifically, to generating a feedback signal indicating voltage across the inductor of the LED lamp.
2. Description of the Related Art
LEDs are being adopted in a wide variety of electronics applications, such as architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, and flashlights. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
The use of LEDs in lighting applications is expected to expand, as they provide significant advantages over incandescent lamps (light bulbs) in power efficiency (lumens per watt) and spectral quality. Furthermore, LED lamps represent lower environmental impact compared to fluorescent lighting systems (fluorescent ballast combined with fluorescent lamp) that may cause mercury contamination as a result of fluorescent lamp disposal.
However, conventional LED lamps cannot be direct replacements of incandescent lamps and dimmable fluorescent systems without modifications to current wiring and component infrastructure that have been built around incandescent light bulbs. This is because conventional incandescent lamps are voltage driven devices while LEDs are current driven devices, thus requiring different techniques for controlling the intensity of their respective light outputs.
Many dimmer switches adjust the RMS voltage value of the lamp input voltage by controlling the phase angle of the AC-input power that is applied to the incandescent lamp to dim the incandescent lamp. Controlling the phase angle is an effective and simple way to adjust the RMS-voltage supplied to the incandescent bulb and provide dimming capabilities. However, conventional dimmer switches that control the phase angle of the input voltage are not compatible with conventional LED lamps, since LEDs, and thus LED lamps, are current-driven devices.
One solution to this compatibility problem uses an LED driver that senses the lamp input voltage to determine the operating duty cycle of the dimmer switch and reduces the regulated forward current through an LED lamp as the operating duty cycle of the dimmer switch is lowered. In some cases, the LED driver delivers power to the LED lamp across a transformer, isolating the output of the LED lamp from the input. To regulate the current through the LED, the LED driver receives feedback about an output voltage or current through the LED. Many LED drivers sense the output using an auxiliary winding on the primary side of the transformer. However, sensing the output voltage via an auxiliary winding adds complexity to the LED driver, increasing both the cost and the size of the LED driver.